Lithium secondary batteries comprise a positive electrode and a negative electrode comprising materials capable of reversibly intercalating lithium ions, with an organic electrolyte or a polymer electrolyte injected between the positive electrode and the negative electrode. The battery produces electrical energy as a result of a redox reaction upon intercalating/deintercalating the lithium ions.
The positive active material for the lithium secondary battery employs a chalcogenide compound such as a metal composite compound of LiCoO2, LiMnO2, LiMn2O4, LiNiO2, or LiNi1-xCoxO2 (where 0<x<1).
The negative active material typically comprises a lithium metal, but a battery short circuit may occur in a battery using such a material as the negative active material due to dendrite formation. Therefore, the lithium metal may be substituted with a carbonaceous material. Crystalline carbon such as natural graphite or synthetic graphite and amorphous carbon such as soft carbon or hard carbon have been mentioned for this purpose. While amorphous carbon has a very large capacity, it may cause problems in that it is hard to reverse intercalation during the charge and the discharge. Crystalline carbon, when used for the negative active material, is generally in the form of a natural graphite since it has a high capacity with a theoretical limit capacity of 372 mA h/g, but it has a problem in that the cycle life of the resultant battery is remarkably degenerated.
The negative active material for a negative electrode for a lithium secondary battery is prepared by mixing carbon materials and a binder, and a conductive agent if required, and agitating them to provide a slurry. Then, the slurry is coated on a metal current collector and dried to form a negative electrode. The negative active material is further compressed onto the current collector in order to obtain a uniformly thick electrode plate and increase the capacity of the electrode plate. However, the carbon material tends to crack upon compressing the electrode plate, so the electrode plate may lose its uniformity, which causes the reaction with the electrolyte to lose uniformity and the life of the electrolyte plate to degenerate. Furthermore, if a crack develops, an edge portion thereof is exposed by the pulverized crack so as to increase a side reaction with the electrolyte, and the viscosity of the electrolyte is remarkably increased at a low temperature such as −20° C. because the electrolyte is soaked into the fine cracks by capillary action and the electrolyte taking part in the battery reaction is decreased. Thereby, the discharge characteristic at the low temperature is degenerated.